1. Field of the Invention
This invention concerns displacement transducers and more particularly displacement transducers of the electro-optic type in which optical gratings are utilized to generate a shadow pattern (known as a Moire fringe) which shifts with relative movement between the two gratings. This shifting pattern is detected by means of photodetectors which generate electrical signals which are amplified and processed to produce digital pulse trains corresponding to the direction and extent of relative movement between the gratings.
2. Description of the Prior Art
Moire fringe displacement transducing arrangements as described in U.S. Pat. Nos. 2,886,717 and 2,886,718, and as described above have for various reasons enjoyed considerable success in the context of coordinate measuring machines, such as disclosed in U.S. Pat. Nos. 3,774,311; 3,757,423; 3,403,448. These machines have usually taken the form of a probe shaft supported for linear movement along a first (Z) axis, the probe shaft support being supported for linear movement along a second (Y) axis orthogonal to the first axis on a carriage which is in turn supported for movement along a third (Y) axis orthogonal to the first and second axes.
Movement along each axis is detected by means of the Moire fringe transducer, i.e., a scale grating is affixed to each of the supporting members aligned with movement therealong and an index piece is carried by the member supported and positioned so as to produce the Moire fringe described.
For various reasons known to those skilled in this art, it is often desirable for the probe shaft to support a probe which is capable of limited detected movement in the plane orthogonal to its axis independently of the means of movement comprised by the supporting structure described above.
While it would be advantageous to detect such movement by a similar Moire fringe transducer means carried within the probe since the signals produced would be directly usable by existing system electronics, such an arrangement has not heretofore been provided which was sufficiently compact to be carried within the confines of the probe.
For example, the two-axis transducer arrangement shown in U.S. Pat. No. 3,184,600 while producing a pulse type output upon movement of a probe in a plane utilizes a relatively bulky mechanical linkage to maintain proper orientation of the axes, which approach would not be feasible in this context. In addition, the effective control by this linkage in precisely maintaining the orientation of the grid and the index piece carried by the probe is difficult to achieve.
It has also been discovered that in utilizing offset grid patterns as described in U.S. Pat. No. 2,886,717 to produce relatively phased signals for the purpose of detecting direction of movement, if the offset patterns are positioned relative each other in areas offset from each other in a direction aligned with the grating lines, the signal phasing is highly sensitive to slight skewing rotations of the grating pattern such as induced by torques applied thereto. Since such probes are sometimes used in modes tending to apply torsional loads about the probe axis, this sensitivity could create signal errors.
It is, therefore, an object of the present invention to provide a compact multi-axis Moire fringe transducer arrangement suitable for such applications.
It is a further object to provide a grating pattern arrangement utilizing offset line sets which are relatively insensitive to rotation of the pattern relative the grid pattern.